The present invention relates to an ignition apparatus for internal combustion engines, and particularly to an ignition apparatus for internal combustion engines which is capable of preventing the occurrence of erroneous operation caused by noise at the time of starting the starter motor.
FIG. 1 shows a conventional ignition apparatus for internal combustion engines, wherein the positive terminal of a DC power source 1 is connected to the collector of a power transistor 4 via a power source switch 2 which turns on or off the DC power source 1, and via the primary winding of an ignition coil 3.
A wave form processing circuit 5 operates the power transistor 4 upon receipt of a signal from a signal generator coil 6 of a signal generator which generates a signal in synchronism with the revolution of the engine.
One end of the coil 6 is grounded, and the other end thereof is connected to the base of a transistor 5b via a resistor 5a.
The transistor 5b constitutes a Schmitt circuit together with a transistor 5e. Emitters of the transistors 5b, 5e are commonly connected together and are grounded via a resistor 5h.
The collector of transistor 5b is connected to the base of transistor 5e via a resistor 5d, and is further connected to the positive terminal of the DC power source 1 via a resistor 5c and the power source switch 2.
Further, the collector of transistor 5e is connected to the positive terminal of the DC power source 1 via a resistor 5f and the power source switch 2, and is connected to the base of power transistor 4 via a resistor 5g. The emitter of power transistor 4 is grounded.
The secondary winding of the ignition coil 3 is grounded via an ignition plug 7.
FIG. 2 shows signal wave forms at each of the portions of FIG. 1.
FIG. 2(a) shows the power source voltage V.sub.B applied from the DC power source 1 to the wave form processing circuit 5, FIG. 2(b) shows the starter current I.sub.S, FIG. 2(c) shows the signal voltage V.sub.S of the signal generator coil 6, FIG. 2(d) shows the output voltage V.sub.1 of the transistor 5b, FIG. 2(e) shows the primary current of the ignition coil 3, i.e., it shows the collector current I.sub.C of the power transistor 4, and FIG. 2(f) shows the discharge voltage of the ignition plug 7.
The operation of the circuit in FIG. 1 will be described below. First, if the key of a car is manipulated to turn the power source switch 2 on at a moment t.sub.0 shown in FIG. 2, the power source voltage V.sub.B is applied to the wave form processing circuit 5. At this moment, the transistor 5b is rendered nonconductive since the voltage V.sub.S input to the base thereof is zero, and the transistor 5e is rendered conductive, so that the output voltage V.sub.1 thereof assumes a high level as shown in FIG. 2(d).
Next, if the starter motor is started near a moment t.sub.1, the starter current I.sub.S rises (FIG. 2(b)). The starter current I.sub.S is of the order of several hundred amperes, has a steep wave form, and generates a considerably intense magnetic field near the cable of the starter motor. If the signal generator coil 6 is located in the region of the magnetic field, a voltage proportional to the rate of change of the starter is induced in the signal generator coil 6 as a noise voltage Vn in synchronism with the starter current I.sub.S.
If the noise voltage Vn exceeds the turn-on voltage Von of the transistor 5b in the wave form processing circuit 5, i.e., in the prestage of the Schmitt circuit, the Schmitt circuit is triggered.
That is, the transistor 5b is turned on at a moment t.sub.1 in FIG. 2, and the output voltage V.sub.1 assumes a low level. Therefore, the transistor 5e is turned off, the power transistor 4 which serves as a switching element is turned on, and a collector current In flows.
As the noise voltage Vn decreases and becomes lower than the turn-off voltage Voff of the transistor 5b at, for example, a moment t.sub.2, the transistor 5b is turned off again, and the output voltage V.sub.1 thereof assumes a high level. Therefore, the transistor 5e is turned on, the power transistor 4 is turned off, the collector current In is interrupted at the moment t.sub.2, and a discharge voltage Vg appears at the ignition plug 7.
As the starter current I.sub.S is supplied, the starter turns the engine with its output torque, and a first normal signal V.sub.S is generated in the signal generator coil 6 near a moment t.sub.3. Every time the signal voltage V.sub.S crosses the turn-on voltage Von and the turn-off voltage Voff of the transistor 5b, the transistors perform the same operations as those mentioned with reference to the moments t.sub.1 and t.sub.2. That is, the collector current I.sub.C is permitted to flow at the moments t.sub.3, t.sub.5, and t.sub.7, and is interrupted at the moments t.sub.4, t.sub.6 and t.sub.8, so that spark discharge takes place normally at the ignition plug 7.
According to the conventional ignition apparatus for internal combustion engines as described above, the wave form processing circuit 5 is erroneously operated and the spark discharge takes place at the ignition plug 7 at the moment t.sub.2 which is earlier than the normal moment t.sub.4 of ignition, in case noise voltage Vn is induced in the signal generator coil 6 due to the magnetic field established by the starter current I.sub.S. Therefore, a counter torque is generated in the engine, making it difficult to start the engine.
To eliminate this drawback, it has heretofore been attempted to place the cable in which the starter current flows remote from the signal generator 6. This sort of arrangement, however, is often restricted by the requirements of the layout of various cables.
As other prior art, Japanese patent application Laid-Open No. 53-49638 (published on May 6, 1978) discloses an ignition apparatus for internal combustion engines, according to which the flow of current to the ignition coil is inhibited by forcibly turning on the input transistor for a predetermined period of time at the time of starting the engine, so that the transistor will not respond to noise that enters during that period. Further, Japanese patent application Laid-Open No. 52-87536 (published on July 21, 1977) discloses a selective threshold ignition circuit according to which the DC threshold level is changed at the time of starting or the running of a car, in order to improve noise-resistant performance during the period of running.